docs: def run_as(self, identifiers):
          """
          :type identifiers:  subject_abcs.IdentifierCollection
          """
          if (not self.has_identifiers):
              msg = ("This subject does not yet have an identity.  Assuming the "
                     "identity of another Subject is only allowed for Subjects "
                     "with an existing identity.  Try logging this subject in "
                     "first, or using the DelegatingSubject.Builder "
                     "to build ad hoc Subject instances with identities as "
                     "necessary.")
              raise ValueError(msg)
          self.push_identity(identifiers)